This invention relates to signal conversion circuits for photosensor arrays and, more particularly, to a new and improved circuit for converting analog photoelectric signals emitted by each photosensor element of a photosensor array into variable pulse width signals and then into digital signals, thereby facilitating subsequent data processing.
With recent improvements in semiconductor techniques, miniature photosensor arrays with integrated photosensor elements have been developed, and have found widespread application. However, a shortcoming of such integrated arrays is that the brightness range of an image received on the array may be too large and variable to permit a circuit which receives the photoelectric signals to cope satisfactorily therewith. In other words, the image received on each photosensor element in the array may have a brightness range of about 1:10.sup.6. Therefore, the minimum output value of the photosensor will be only about 1 .mu.v while the maximum output will be about 1v. It is difficult or impossible for an ordinary electronic circuit to handle signals having such a large dynamic range, and, accordingly, the full information content in the photoelectric output of such sensors cannot be utilized.
One way to overcome this difficulty is to convert the photoelectric signal into a form of pulse signal with a variable pulse width and then use the pulse signal for further information processing or further digitize that signal for information processing. According to this technique, the pulse height of the pulse signal can be kept constant regardless of the intensity of lighting, thereby facilitating handling by an electronic circuit, and the information in the variable pulse width signal can easily be converted into a digital value having a corresponding number of bits, for example, by timing the pulse width with a clock pulse, thus facilitating information processing. Such circuits, however, are subject to conversion errors of the type described hereinafter.
Accordingly, it is an object of this invention to provide a signal conversion circuit for an integrated photosensor array having low conversion error whereby pattern data produced from photoelectric signals are obtainable without deterioration of the original pattern data even if there are manufacturing errors in circuit elements.